The invention relates to wastewater treatment systems, and more particularly to pre-thickened aerobic digester systems.
Enviroquip's U.S. Pat. No. 5,972,220 describes a pre-thickened aerobic digester (P.A.D.) G system. This system eliminated the need to turn oxygen on and off in the digesters, by performing denitrification in the gravity thickener, and enabled class B sludge to be achieved in a lesser time than in prior P.A.D. systems. Although the system works very well, there can be several problems: if the wastewater comes in from a sequential batch reactor, or SBR, or septage, the batches come infrequently, e.g. eight times per day (assuming from two SBRs), or less frequently in the case of septage, which is concentrated. The basic P.A.D. G system is designed for receiving up to 24 batches per day.
Another problem is that the gravity thickener may need to be taken out of service, for mechanical repairs, cleaning, etc. There was no way to do this prior to the new development described herein, for the P.A.D. G system.
A third issue is low flow in a system designed for a much larger flow. This is typical of a system designed for future expansion, i.e. to serve a larger area in the future. With a flow rate which is only a fraction of the design flow rate, the P.A.D. G system simply does not receive enough sludge to denitrify the sludge. The thickener is much larger than needed for these circumstances, and the sludge will not settle adequately. Sludge will tend to float up to the surface, and the supernatant is not sufficiently clear; only about 1% to 1½% sludge concentration is possible in many such situations.